The undulator is a light source which emits polarized radiation. To this end, the undulator is positioned along and/or around an accelerator track. The undulator, via the portion of its magnetic field near the axis, acts upon the electrically charged particle beam passing therethrough. Due to its speed {right arrow over (v)}, the particle beam interacts with the undulator magnetic field {right arrow over (B)} in the region of the undulator according to the relation {right arrow over (v)}×{right arrow over (B)}, a deflecting magnetic field of a certain strength; i.e., a deflecting force, the Lorentz force FL=e {right arrow over (v)}×{right arrow over (B)}. Undulators are used, in particular, to generate short-wave electromagnetic radiation, mainly X-ray radiation, in synchrotrons. The optical axis of the photon radiation emitted from the undulator is tangential to the particle beam axis.
German document DE 103 58 225 describes an undulator and method of operation thereof. The introductory description of that document includes a description of the prior art and of the physical idea underlying the construction of a special undulator which includes at least two subassemblies. The described undulator, by means its magnetic field and the particle beam passing therethrough, generates synchrotron radiation; each partial undulator including a superconductive material which, when energized with a current, generates an undulator field which is perpendicular to the direction of the current; and the superconductive material in the individual partial undulators being disposed in such a manner that the undulator fields generated by the partial undulators are not parallel to each other. In addition to the explanation of the physical principles of construction, the disclosure describes an undulator coil having two sections of equal length: an inserted planar section and a surrounding helical section. FIG. 2 of DE 103 58 225 shows the planar-helical undulator having two identical coils whose planar and helical sections have an equal number of winding chambers and windings, and in which the planar section is coincidently surrounded by the helical section. There, the planar and helical sections are identical in length. A superconducting planar-helical undulator with electrically switchable helicity is described by U. Schindler in scientific report No. FZKA 6997 of the Karlsruhe Research Institute in Germany, in particular in Chapter 4, entitled “Superconducting Undulators”. In section 4.4 “Technical Implementation” and to A.4. “Engineering Drawings”, pages 45 and 46 the winding technique is illustrated, including the overpass and underpass of the winding wire (FIG. 4.9, of the electrical series connection of the wound winding chamber and the antiparallelism of the axes of the magnetic fields of successive, wound winding chambers of the respective section of a coil. The configurations of a planar and a helical coil form is illustrated in FIG. 4.10 and FIG. 4.11, and on pages 45 and 46. One coil of the undulator is obtained from the other by rotation through 180° about the undulator axis. This planar-helical undulator is capable of generating X-ray radiation with electrically variable polarization and is configured as follows:
Two coils of the same type are located opposite and equidistant from one another with respect to the undulator axis and are at the same distance from the undulator axis which, in the installed condition, forms part of the synchrotron beam axis. A coil including two sections, namely a helical section and a planar section, the planar section being inserted and positioned in the helical section. The sections each include a coil form made of non-magnetic material, and winding chambers which are milled into the coil form about the coil axis. The planar coil form axis coincides with the helical coil form axis, both forming, or lying on, the coil axis.
The coil axis extends through the planar winding chambers at a right angle thereto, while similarly the helical coil axis extends through the helical winding chambers at an angle of 45° thereto. The distances between the successive winding chambers, the structural period length γb, are the same in both coil forms. The undulator axis and the coil axes are parallel to each other and extend in one plane, the plane of axes.
The bottom of each winding chamber, the winding base, is convex and, more specifically, circular in the case of the inserted planar section. The point in the winding base at which the radius of curvature is largest or, in the case of the helical section, the region of largest radius of curvature, is closest to the undulator axis in central relationship to the plane of axes. The two sections of a coil are positioned relative to each other such that a planar winding chamber and a surrounding helical winding chamber at the same axial location intersect each other twice in the plane of axes in skew relationship to each other, and that they are closest to each other at their respective regions that are closest to the undulator axis. There, the maximum radius of curvature of the winding chamber of the inserted section is no greater than that of the winding chamber of the surrounding winding chamber, the two winding chamber planes forming an angle α of 45°.
A section includes an inlet region and an outlet region for the winding wire on the shell in the region of one end face, and a winding wire connection on the shell in the region of the other end face, the winding chamber region being located therebetween. A section is made in one piece or, for a small number of winding chambers, it is composed of the two end face regions or, for a larger number of winding chambers, it is composed of the two end face regions and at least one chamber region located therebetween; the at least two section components being joined by axial connecting elements in a section-forming manner.
The winding wire is a normal electrical conductor or a technical superconductor and is used to wind a section under a permanent preset tension, always in the same winding direction, as follows: A first length of winding wire extends in a form-fitting, embedded manner from the winding wire inlet across the shell to the winding base of the first winding chamber and passes under the same in a form-fitting, embedded manner. Then, it penetrates the shell to the next, second winding chamber where it extends to the winding base and is wound up therein. From there, the winding wire penetrates the shell to the next, third winding chamber where it extends to the winding base and passes under the same in a form-fitting, embedded manner. Further, the winding wire penetrates the shell to the winding base of the next, fourth winding chamber in which it is wound up in the same direction as before. This procedure is continued until the last even-numbered winding chamber is reached. If this is the last winding chamber, the winding wire is wound up therein and connected to the winding wire connection or, if the last winding chamber is odd-numbered, the winding wire passes under this last winding chamber and connects to the winding wire connection.
A second length of winding wire extends in a form-fitting, embedded manner from the winding wire outlet across the shell to the winding base of the first winding chamber and is wound up therein in the same direction as in the even-numbered winding chambers. Then, it penetrates the shell to the second winding, passes over the same, then penetrates the shell to the third winding chamber where it extends to the winding base and is wound up therein in the same direction as before. Then, it penetrates the shell to the fourth winding chamber, passes over the same, then penetrates the shell to the fifth winding chamber where it extends to the winding base and is wound up therein. This procedure is continued until the last even-numbered winding chamber is reached, from where the winding wire passes over the even-numbered winding and connects to the winding wire connection. The underpasses and overpasses, as well as the conductor terminals and connections, are arranged in the coil form region facing away from the undulator axis. Since the two lengths of winding wire are connected to one another, the windings are electrically in series, but when energized, they generate magnetic fields whose successive axes extend in opposite directions; in the case of the helical section, they extend in opposite parallel directions. The number of windings in the winding chambers of a section is constant.
Moreover, means are provided which allow the current levels applied to the superconducting material in the individual partial undulators to be adjusted independently of one another, as a result of which the undulator field resulting from the superposition of the undulator fields generated by the partial undulators determines the polarization direction of the synchrotron radiation. To this end, a first partial undulator is disposed such that its first undulator field is substantially perpendicular to the direction of the particle beam, and a second partial undulator is disposed such that its second undulator field has a component different from zero in the direction of the first undulator field and another such component in a direction which is substantially perpendicular to the direction of the first undulator field and substantially perpendicular to the direction of the particle beam.
In the FZKA 6997 report, the section-dependent polarization is described in detail for the situation where the sections are of equal length, the planar section is located centrally and has circular winding chambers, and where the number of windings in the winding chambers is constant in both sections, respectively, and thus, the described section-dependent polarization is directly transferable to the zones of the planar-helical undulator having both sections. The portions of the planar-helical undulator that have only the two planar sections generate only linearly polarized light. Conversely, the portions of the planar-helical undulator that have only the two helical sections generate only light having generally elliptical polarization
The technical problem consists in the manufacture of an undulator and, thus, in the implementation of the windings of such an undulator. Superconducting undulators, in particular, make it possible to achieve high magnetic field strengths and high field gradients, enabling reliable operation without degradation or spontaneous transition from superconduction to normal conduction, which is known as quenching or quenching effect. The physics described in German document DE 103 58 225 gives rise to the object of providing an undulator which is made of electromagnetic components and allows the desired polarization of the light emitted from the undulator to be adjusted only by changing the current in the conductor sections that generate the undulator magnetic field, and not by means of mechanically/locally moved undulator portions. The above-cited scientific report No. FZKA 6997 describes the technical solution for the purely linear, circular, generally elliptical polarization, and provides structural details of the coil forms. However, the planar-helical undulator described therein is only capable of producing one of the three aforementioned types of polarization in the emitted beam, depending on the setting of the currents in the two coils, with a polarization of the photon radiation emitted therefrom which is electrically completely variable over 360°. It is technically difficult to produce a polarization that differs from zone to zone.